1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the zoom lens. The zoom lens may be suitable for application to television cameras for broadcasting, video cameras, digital still cameras, and cameras for silver-halide photography, among others.
2. Description of the Related Art
A zoom lens with a high zoom ratio and high optical performance is desired for image pickup apparatuses such as television cameras, cameras for silver-halide films, digital cameras, and video cameras. As a zoom lens with a high zoom ratio, there is known a positive-lead telephoto zoom lens including four lens units as a whole in which a lens unit disposed closest to the object has a positive refractive power. For example, there is known a zoom lens with four lens units including a first lens unit with a positive refractive power for focusing, a second lens unit with a negative refractive power for magnification variation, a third lens unit with a positive refractive power for correction of image plane variation, and a fourth lens unit with a positive refractive power for image formation. There have been proposed zoom lenses with four lens units that achieve a wide field angle and a high zoom ratio (U.S. Pat. No. 6,956,704, U.S. Pat. No. 7,583,449, and U.S. Pat. No. 7,903,345).
U.S. Pat. No. 6,956,704 discloses a zoom lens with a photographing field angle at the wide-angle end of 60.93° and a zoom factor of about 96. U.S. Pat. No. 7,583,449 discloses a zoom lens with a photographing field angle at the wide-angle end of 69.02° and a zoom factor of about 106. U.S. Pat. No. 7,903,345 discloses a zoom lens with a photographing field angle at the wide-angle end of 62.86° and a zoom factor of about 120.
Image pickup apparatuses such as television cameras, cameras for photography, and video cameras are preferable when their entire system is made small in size and light in weight, in addition to having high optical performance. Accordingly, it is highly desirable that a zoom lens with a high zoom ratio and wide field angle also be made compact and light weight. In a zoom lens with a high zoom ratio, the size and weight of the lens unit on the object side significantly affects the operability of the camera lens. Therefore, it is becoming particularly important to reduce the size and weight of the lens unit on the object side.
In order to achieve a high zoom ratio, in general, it is necessary to secure a long movable range for the lens unit for magnification variation, which tends to increase the effective lens diameter of the first lens unit disposed closest to the object. In order to achieve a wide field angle, meanwhile, it is necessary to secure a wide angle of incidence of a ray taken into the lens system, which also tends to increase the effective lens diameter of the first lens unit. In order to suppress an increase in effective lens diameter of the first lens unit while increasing the field angle and the zoom ratio at the same time, it is effective to reduce the number of lens elements forming the first lens unit and move the rear principal point of the first lens unit toward the image plane.
In order to increase the field angle and the zoom ratio while reducing the size of the entire system of the zoom lens with four lens units discussed above, in particular, it is important to appropriately set the lens configuration of the first lens unit which does not move for zooming.
If the lens configuration of the first lens unit is inappropriate, it is difficult to obtain a zoom lens with a wide field angle and a high zoom ratio while reducing the size of the entire system.